Semiconductor gas sensors are used to detect the presence of a particular gas or gasses in an environment to which the sensor is exposed. A common type of gas sensor is a metal oxide semiconductor (MOS) gas sensor. MOS gas sensors, which are also referred to as “thick-film” gas sensors, typically include a heating element and a gas-sensitive portion located between two electrodes. The heating element is activated to heat the gas-sensitive portion to a temperature that is suitable for detecting a target gas. The gas-sensitive portion is a polycrystalline thick-film that is configured to undergo a change in optical transmittance, electrical conduction, and/or ionic conduction in the presence of the target gas. The change of the gas-sensitive portion is detected by an external circuit that is electrically connected to the gas sensor.
Two common types of thick film MOS gas sensors are carbon monoxide sensors and alcohol sensors. Carbon monoxide sensors are used in both automotive and home applications. For example, carbon monoxide sensors are useful for determining the presence, absence, or concentration of carbon monoxide in automotive exhaust products. Carbon monoxide sensors are used in the home for detecting unsafe levels of carbon monoxide. Alcohol sensors are used in applications including automotive fuel systems and breath analyzer devices. In most applications, it is desirable for both types of sensors to be small, inexpensive, accurate, and electrically efficient. It is also desirable for the sensors to quickly determine the concentration of gas in the selected environment.
FIGS. 1 and 2 show part of a gas-sensitive portion 10 of a prior art MOS gas sensor. The polycrystalline material of the gas-sensitive portion 10 includes numerous grains 20. The region of contact between the grains 20 is referred to herein as a grain boundary 22. The grain boundaries 22 are target sites to which molecules of the target gas bind through a process referred to as adsorption. When adsorption of the target gas occurs, the gas-sensitive portion 10 undergoes the above-described change that is detected by the external circuit.
Chemisorption is one type of adsorption that may occur at the grain boundaries 22 in the presence of the target gas. To illustrate the effects of chemisorption, FIG. 1 includes a graph showing an electrical potential barrier at the grain boundary 22 in an environment of air containing oxygen molecules. For an electron 30 to move through the grain boundary 22, it requires enough energy to overcome the potential barrier, which defines a reference magnitude measured in electronvolts (eV). A combination of the potential barriers of all/most of the grain boundaries 22 in the gas-sensitive portion 10 contributes to an electrical resistance of the gas-sensitive portion.
In FIG. 2, the exemplary grain boundary 22 is shown in the presence of molecules of a reducing gas. Chemisorption of the reducing gas has caused a reduction in the magnitude of the potential barrier due to donor electrons from the reducing gas. When the potential barriers are combined, the overall electrical resistance of the gas-sensitive portion 10 is reduced due to the reduction in the magnitude of at least some of the potential barriers at the grain boundaries 22 at which reduction has occurred. The exemplary reduction in electrical resistance of the gas-sensitive portion 10 is detectable by the external circuit connected to the gas sensor as being indicative of the presence of a target gas. Although not shown, in the presence of an oxidizing gas, the magnitude of the potential barrier increases, thereby resulting in an increase in the electrical resistance of the gas-sensitive portion 10, which is also detectable by the external circuit connected to the gas sensor as being indicative of the presence of a target gas.
Heterogeneous catalysis is another process that may occur at the grain boundaries 22, depending on the type gas near the gas-sensitive portion 10. One example of heterogeneous catalysis, referred to as carbon monoxide (CO) oxidation, results in the oxidation of a carbon dioxide (CO2) molecule, due to the presence of a carbon monoxide molecule and an oxygen molecule located near one of the grain boundaries 22 of the gas-sensitive portion 10. Heterogeneous catalysis, in at least some instances, results in the change of the gas-sensitive portion 10, which is detectable by the external circuit connected to the gas sensor as being indicative of the presence of a target gas.
The change in optical transmittance of a thick film MOS gas sensor in the presence of the target gas is also a catalytic reaction. Optical thick film gas sensors are found, for example, in carbon monoxide detectors and typically include an optical gas sensor and a read out circuit. The gas sensor includes a gas sensitive portion formed from a thick film of tin dioxide and nickel oxide, for example, that has been heat treated (annealed) at approximately 500° C. The read out circuit is a circuit that is configured to heat the thick film to an operating temperature and to monitor the optical transmittance of the heated thick film, which varies based on the concentration of carbon monoxide in the environment to which the detector is exposed. As shown in FIG. 3, at the four illustrated operating temperatures, the optical transmittance of the thick film at a wavelength of 650 nm steps to a peak value between approximately two hundred to four hundred seconds after being exposed to an environment having 1 vol % carbon monoxide in air.
When the heating element of the typical MOS gas sensor is activated, other portions of the gas sensor are heated in addition to the gas-sensitive portion. For example, if an intermediary layer is located between the heating element and the gas-sensitive portion, then the heating element heats the intermediary layer in addition to heating the gas-sensitive portion. Furthermore, if the heating element is positioned in contact with or in proximity to a base layer, a substrate layer, or a handle layer, then heat energy from the heating element may undesirably/unnecessarily be transferred thereto. Additionally, since the gas-sensitive portion of a MOS gas sensor is a “thick-film,” heating of the gas-sensitive portion has an associated time constant that may be of longer duration than desired. Accordingly, in the typical MOS gas sensor, energy consumed by the heating element is used to heat portions of the gas sensor that are not desired to be heated, and heating the gas-sensitive portion may consume more time than desired.
Thick film MOS gas sensors are useful for sensing a target gas, but are difficult and time consuming to fabricate, especially when the gas sensitive portion includes multiple layers of mutually catalytic materials. Additionally, thick film MOS gas sensors, especially optical-based MOS gas sensors, are larger and slower than is suitable for some applications, such as sensing the presence alcohol. Furthermore, thick film MOS gas sensors consume significant electrical power when being heated to an operating temperature. Therefore, for at least some of the above-described reasons, further developments in the area of gas sensors are desirable.